This invention relates to a power source device for operating an electronic circuit for monitoring quantities of electricity flowing through a circuit breaker.
FIGS. 2 and 3 show a prior-art power source device for a circuit breaker disclosed in Japanese Patent Application Laid-open No. 150920/1975. Referring to FIG. 2, which is a block diagram of the power source device an input current Im is a rectified current of the current flowing through each phase of the circuit breaker. A current sensing circuit 715 applies the input current Im to a switching circuit 710, and it converts the current Im into a voltage Vc and applies the voltage to a microcomputer 154 as a current sensing signal. The switching circuit 710 is controlled by a voltage sensing circuit 708 sensing the voltage Vc to change the output of the current sensing circuit 715 into the voltage by switching between contact 1, which is connected to an energy storing capacitor 704, and contact 2, which is connected to a bypass circuit 714. The energy storing capacitor 704 smooths the input current Im and stores the energy in the form of the voltage Vc. A D.C./D.C. converter 706 is connected to the energy storing capacitor for obtaining a constant voltage output on the basis of the voltage Vc stored by the energy storing capacitor 704.
If the output voltage Vc of the energy storing capacitor 704 based on the input current Im is not greater than a predetermined voltage, for example, 39 V, the voltage sensing circuit 708 throws the switching circuit 710 to the contact 1, and the input current Im charges the energy storing capacitor 704. When the output voltage Vc exceeds the predetermined voltage, the switching circuit 710 is changed-over to the contact 2, so that the input current Im is bypassed by the bypass circuit 714. An example of the bypass circuit 714 is shown in FIG. 3. In the figure, the bypass circuit 714 is a chopper circuit constructed of an FET. As described above, the voltage sensing circuit 708 receives the output Vc and has a hysteresis characteristic to operate the switching circuit 710 on the basis of the predetermined voltage. The D.C./D.C. converter 706 is also controlled by the voltage sensing circuit 708 so as to turn ON and OFF for supplying and interrupting a voltage on the basis of the output voltage Vc and predetermined voltages (for example, ON at 37 V and OFF at 33 V).
Since the prior-art power source circuit for the circuit breaker provides a voltage on the basis of the switching between the ON and OFF states of the D.C./D.C. converter and the charging and discharging of the energy storing capacitor, a capacitor of high capacitance and high withstand voltage is required. In addition, a wide variation in the output impedance of the current sensing circuit due to the charging and discharging of the energy storing capacitor and the switching of the switching circuit 710 (in general, the bypass circuit is of low impedance) effects the accuracy of the detected current.